Here is an example of a typical chart pattern everyone knows, the head-and-shoulders top. It has not exactly fully formed yet, but one can extrapolate the neckline since the right shoulder has already turned.
The problem is, if one trades by the "rules" of the pattern, one would short the downward violation of the neckline, setting a stop at the high of the right shoulder. Yet because this pattern is so well known, you can almost be certain that there will be a wave of selling at the break of the neckline; so people begin to anticipate the pattern and start selling/shorting in anticipation it. This puts those traders who are still trading old patterns by the book at a big disadvantage -- those who are already short are using the neckline break as a "test" to see if there will be any buyers there. Because if one were bullish and wanted to buy this security/index, he or she would most likely wait until after the neckline break to buy into supply to capture the best price.
The crux of the matter is: are the majority of future sellers of this security just selling based on pure technicals? If so, it would not be difficult to find the level where the most buy-stops would be located, in order to shake out/squeeze the "weak-hands" (technical buyers and sellers are by definition weak-hands, as they are price or pattern motivated and typically more bullish at higher prices and more bearish at lower prices). So if you are one of those people who would sell because of some known pattern, at least be aware that the position may be "crowded", and that others in that crowd may not have the same conviction that you may have. In many cases, that is the main reason why your stop would eventually get hit.